Device and method for monitoring the particle contamination in flowing hydraulic fluids

ABSTRACT

A device for monitoring particle contamination in flowing hydraulic fluids includes a mechanism for particle counting and particle sizing. Further, a method for monitoring particle contamination in flowing hydraulic fluids: determines fluid flow velocity; counts particles in the hydraulic fluid passing the light barrier for a fixed period of time; and obtains particle size distribution by using a range of different trigger levels. The monitoring device is insertable into an A/C hydraulic system to enable an online-monitoring of degradation of fluid quality during normal flight operations or on the ground. The device and method help lower costs for A/C maintenance and increase A/C availability since necessary service actions can be scheduled strategically.

BACKGROUND OF THE INVENTION

The present invention relates to a device for monitoring the particlecontamination in flowing hydraulic fluids according to the preamble ofclaim 1 and a method for monitoring the particle contamination accordingto claim 7.

The quality of hydraulic fluids is essential for ensuring the properoperation of safety-critical subsystems in aircrafts such as flaps,slats, landing gears, etc. Contamination of the hydraulic fluid canseverely damage mechanical components in the hydraulic system.Therefore, a stringent contamination control is required at all levelsof maintenance to ensure flight safety and the highest degree ofhydraulic system readiness.

Contamination present in an operating hydraulic system is normallyoriginated at several different sources. Maintenance malpracticesintroduce large amounts of external contaminates. Wear and chemicalreactions are also factors in hydraulic system contamination. The typesof contamination are generally classified as organic, metallic solids,non-metallic solids, foreign fluids, air and water.

Organic solids are produced by wear, oxidation or polymerization. Minuteparticles of O-rings, seals, gaskets and hoses are present due to wearor chemical reactions.

Metallic contaminates are almost always present in hydraulic systems.These particles are the result of wear and scoring of bare metal partsand plating materials such as silver and chromium.

Inorganic or nonmetallic solids include dust, paint particles, dirt andsilicates. These particles are often drawn into a hydraulic system fromexternal sources. Disconnected hoses, lines and components are entrypoints. The wet surface of a hydraulic piston shaft may also draw somecontaminates past the wiper seals and into the hydraulic system.

Air contamination, dissolved, entrained or free floating can adverselyaffect a hydraulic system. Air entrained hydraulic fluid can causesevere mechanical damage from pump cavitations, system pressure loss orslow or erratic flight control movements.

Water is a serious contaminant of hydraulic systems. Dissolved,emulsified or free water may result in the formation of ice or oxidationor corrosion products of metallic surfaces.

Foreign fluids are generally the result of lube oil, engine fuel orincorrect hydraulic fluid having been inadvertently introduced into thehydraulic system during servicing. The effect of foreign fluids otherthan water depends on the nature of the contaminant. Compatibility ofmaterials of construction, reactions with hydraulic fluid and water, andchanges in flammability and viscosity are all affected. The effects ofsuch contamination may be mild or severe depending on the contaminant,amount (quantity) and length of time it has been in the system.

Contamination in oil is specified from particle count. Two basic methodsare used: Laser based particle count analysis equipment gives directlyinformation on particle sizes (micron=μ) and figures within specifiedsize ranges. The other method utilize filtering an oil sample through avery fine mesh filter paper. The particles on the surface of the filterpaper are then monitored in a microscope and compared to standardcontamination pictures to indicate the degree of contamination.

Instead of specifying particle counts contamination is separated intoclasses defined in two major systems ISO (International StandardOrganization) and NAS (National Airspace Standard). Each class defines arange of counts within an exponential scale. Unfortunately, the twosystems are not identical and can not be converted in simplemathematics. However, some simple guidelines can be given. First of alllet's look at the two systems.

The NAS system divides particles in 5 ranges. Furthermore, the NASsystem specifies different counts within each particle range to score aspecific class. In practice, oil samples will show up to gain almost thesame NAS class rating within the different particle ranges. The systemis designed to match the most common found contamination which hasreally many small particles and fewer big particles. A typical oilanalysis can for example have counts divided in the 5 classes. Theresulting NAS class according to NAS1638 is defined as the particlecount with the highest (worse) score, and only this class is specified.

Currently the quality of hydraulic fluids is generally monitoredoff-line. During A/C service, fluid samples are tapped from thehydraulic system and their quality is assessed with laboratory equipmentoff-line. This procedure is time-consuming and costly. Necessary serviceactions are taken according to fixed schedules rather than on demand.

U.S. Pat. No. 4,323,843 discloses an apparatus for detecting ferrouscontamination in a fluid such as engine transmission lubricant. Theapparatus has two spaced apart electrodes, a magnetic flux extendingbetween the electrodes, the lines of force of which are substantiallyrectilinear, the electrodes being connectable to a circuit for signalingwhen an electrically conductive path is formed from one electrode to theother by metal particles attracted to the flux and formed into anelectrode spanning bridge. For preference the sensor is a hollow plug,which in use is screwed into a transmission housing port, and has a flatend wall in contact with the lubricant in the housing. The plug containsa magnet having both poles disposed towards the plug end wall, a portionof the end wall acting as one electrode. A ferromagnetic disc mountedexternally from the plug end wall, and electrically insulated therefrom,overlies the poles. The disc acts as a second electrode and serves todirect the magnetic flux rectilinearly across the gap between theelectrodes. The electrodes are connectable to circuit means whereby achange in interelectrode resistance may be detected. A major drawback ofthis detection device is the fact that only ferrous contamination can bedetected. Therefore the effectiveness is very limited with respect tothe efforts made and the costs involved.

U.S. Pat. No. 5,754,055 describes a lubricating/hydraulic fluidcondition monitor in which a coaxial microwave resonator is placed in afluid conduit to determine changes in the chemical properties and debrisconcentration is disclosed. Microwave radiation is applied to theresonator for measuring the resonant frequency and resonator Q. Anexternally powered electric or magnetic field is used to alternatelyalign and misalign debris in the fluid while the resonator propertiesare being measured. A logic unit automatically generates tables ofresonant frequency and Q versus resonator mode and external fieldstrength. This set of tables constitutes a fingerprint of the fluidcondition. By matching the fingerprint against a set of fingerprintstaken under known conditions, the condition of the fluid is determined.Changes in the fluid's dielectric constant caused by oxidation or thepresence of water, changes in the concentration and size of conductingparticles from bearing wear, and changes in viscosity all affect thefingerprint; and thus, can be monitored in real time. In a variation ofthe invention, a lumped-circuit resonator printed on a microwave circuitboard is used as the sensor. In a further variation, a transmission-lineresonator printed on a microwave circuit board is used as a sensor. Inyet another variation the resonator is a lumped circuit wave guidestructure through which the fluid flows. In still another variation,time domain reflectometry is used in a transmission line having one endimmersed in the fluid. A major drawback of this solution is the use ofmicrowave frequencies which cause problems for in-flight operation.Especially in the vicinity of fly by wire systems this detection devicecan not be used during operation of an aircraft.

U.S. Pat. No. 4,013,953 discloses an optical oil monitor that measuresparticle contamination in oil by passing light through an oil sample andpicking up the light that is scattered at 90° by the particlecontamination and measures chemical breakdown by the attenuation of thelight passing substantially straight through the oil with a second photosensor. Alternately a sample and a reference are passed between thelight responsive sensors for error correction and calibration so thateach sensor will have an output signal alternating between a samplesignal and a reference signal. The sample and reference are housedwithin a rotor provided with vanes so that it may be driven as a pump bya motor or be driven by fluid flow as a turbine. When the rotor acts asa turbine, the frequency of the light responsive sensors will becorrelated to the fluid flow rate indicated by the turbine turns so thatan appropriate frequency responsive gauge is provided in circuit tomonitor the fluid flow. The peak signals of a peak detector indicatingthe particle count are summed up to provide an output corresponding tothe amount of contamination. Major drawbacks of this device are thecomplex mechanical construction including a turbine sample rotor in thelubrication oil flow as the main constructive part. This may lead tomechanical defects and failure of the entire hydraulic system. Further,no indication of the particle size can be provided by the knownsolution. Finally this known solution is costly and uses relativelylarge space.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a devicefor monitoring small solid particles having a size range from 0.1 μm to100 μm in hydraulic fluids which avoids the drawbacks of the prior art.The monitoring device should be insertable into the A/C hydraulic systemto enable an online-monitoring of the degradation of the fluid qualityduring normal flight operations or on the ground. Further the inventivesolution should help lowering the costs for A/C maintenance and increasethe A/C availability.

These objects are solved by a device having the features of claim 1 andby a method having the features of claim 7. Advantageous embodiments aredescribed in the dependent claims.

The device for monitoring particle contamination in flowing hydraulicfluids according to the present invention is characterized in that itcomprises means for particle counting and particle sizing. The deviceaccording to the present invention allows online-monitoring of ferrousand non-ferrous small solid particles having a size range from 0.1 μm to100 μm in hydraulic fluids and avoids the drawbacks of the prior art.The monitoring device is insertable into the A/C hydraulic system toenable an online-monitoring of the degradation of the fluid qualityduring normal flight operations or on the ground. Further the inventivesolution helps lowering the costs for A/C maintenance and increase theA/C availability since necessary service actions can be scheduledstrategically. For evaluation of the measurement information, a dataprocessing unit can be provided. The data processing unit may includeanalog/digital converters, data storage and a CPU for executing testevaluation software. Such software my include a database storing NAS1638data for data verification. The device according to the presentinvention can help to maintain the level of particulate mattercontamination in A/C hydraulic systems of aircrafts at an acceptablelevel and ensures minimum degradation of the mechanical componentsconstituting the hydraulic systems.

A preferred embodiment of the present invention is characterized in thatthe device comprises an optical sensor and a flow sensor. The opticalsensor may be a photoelectric slot sensor having a light beam diameterof 100 μm. To determine the particle size and number, a light barriertechnique can be used utilizing the shadowing effect upon particletransit to calculate the particle number and particle size.

A preferred embodiment of the present invention is characterized in thattwo or more optical sensors having different light beam diameters areprovided. The light beam diameters may start from 10 μm or 20 μm andrange up to a diameter of 100 μm or 150 μm.

A further preferred embodiment of the present invention is characterizedin that, the flow sensor is an ultrasonic transducer. The fluid flowmeasured in flight operation is produced by the hydraulic pump system ofthe aircraft and might differ because of the particular flightsituation. This goes especially for non steady flight situation wheresometime high G-forces apply. Further, to be able to use an ultrasonictransducer, the fluid temperature should also be measured to calculatethe speed of sound in this fluid. In case of particle contaminationmonitoring on the ground, the flow velocity may be adjusted to a knownvalue which makes additional flow rate detection unnecessary.

The method for monitoring particle contamination in flowing hydraulicfluids according to the present invention comprises the following steps:

-   -   Determining fluid flow velocity;    -   Counting of particles in hydraulic fluid passing the light        barrier for a fixed period of time;    -   Obtaining particle size distribution by using a range of        different trigger levels.

This method can be advantageously used with a device for monitoringparticle contamination in flowing hydraulic fluids comprising means forparticle counting and particle sizing. Further a data processing unitshould be provided to allow storage and evaluation of the data using arespective evaluation software.

According to a preferred embodiment of the present invention repeatedmeasurements of particle contamination are conducted to allowextrapolating the time at which a critical contamination level is likelyto be reached. With such information necessary maintenance actions canbe scheduled strategically, i.e. together with other maintenanceactions.

According to a preferred embodiment of the present invention signal sizeand signal duration are attributed to particle size. The signal outputvoltage, which is triggered by particles passing through the opticalsensor, can be visualized and evaluated according to the shape (signalwidth) and the signal amplitude.

According to another preferred embodiment of the present invention thecumulative size spectrum and the differential size spectrum are used forcalculating the particle size. This calculation can be verified by usingthe statistical data according to NAS1638.

According to still another preferred embodiment of the present inventionthe range of detectable particle diameters is pre-set to 10 μm to 100μm. This avoids misinterpretation of noise signals as particle counts.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention willbecome more clear from the following detailed description of preferredembodiments of the present invention shown in the attached drawingwherein:

FIG. 1 shows a schematical view of a preferred embodiment of a deviceaccording to the present invention;

FIG. 2 a-c are an explanatory depiction of the functional principle ofthe device according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

As shown in FIG. 1, the monitoring device 1 consists of a metal casing 2containing one or more light barriers 6 for the counting of particles aswell as an ultrasonic flow sensor A, B with a built-in temperaturesensor (not shown) for enabling a quantitative measurement of theparticle size.

The metal casing 2 is constructed as a conduit having in general acircular cross section and bypassing the main hydraulic tubing. Thecasing 2 therefore has a U-shaped sectional view, as can be taken fromFIG. 1. The bypass fluid F enters the casing 2 on the left verticalU-leg and is guided in the horizontal portion of the casing 2 in thecenter of which the photo sensors of the light barriers 6 are located.Further, on the outer left end of the horizontal U-portion of the casing2 an ultrasonic transducers A is located and on the outer right end ofthe horizontal U-portion of the casing 2 an ultrasonic transducers B islocated. The flowing hydraulic fluid F is leaving the monitoring device1 on the right vertical U-leg to re-enter the hydraulic main circuit.

The two light barriers 6 are positioned in a way that the light pass 4is perpendicular to the direction of the fluid F flow. Optical windows 8separate optical emitters 3 and optical detectors 5 from the fluid F.The light barriers 6 consist of the optical emitter 3, which in thepresent prototype is a red-light-LED, and the optical detector 5, whichis in this embodiment a silicon photodiode. The sensitive area of thephotodiode is determined by the area of an aperture.

The size of the aperture is adjusted to the maximum size of particles tooptimize the shadowing effect of a single particle. In the preferredembodiment according to FIG. 1, the left hand light barrier 6 has alarger light beam diameter than the right hand light barrier 6. In thepresent embodiment the larger light beam diameter is 100 μm and thesmaller diameter is 50 μm.

The light barriers 6 are separated from the fluid by means of opticalwindows 8. In the present embodiment the optical windows are two planewindows positioned on opposite sides of the fluid carrying pipe 9. Thewindows are sealed by O-rings and fixed by wheel flanges. The LEDemitters and photodiode receivers are positioned inside opposite wheelflanges.

The ultrasonic transducers A, B are positioned to emit coaxially withthe fluid F flow. The (not shown) temperature sensor is in directcontact with the fluid to allow a measurement of the fluid temperature.The ultrasonic flow sensor consists of two piezo-electric transducerswhich can either work as a generator G or as a receiver R of ultrasonicsound waves. Both transmitters are positioned coaxially along thedirection of the fluid F flow. Both transducers are in direct contactwith the fluid and are sealed by means of O-rings and fixed within wheelflanges.

Following, the function of the monitoring device 1 during operation willbe described. Particles drifting with the fluid F are detected by meansof light barriers 6 which are powered by a constant current 7. LED's areused as light sources and photo diodes are positioned opposite to theLED emitters. The sensitive area of the photodiodes is determined by thesize of mechanical apertures. In case a particle passes the lightbarriers 6, a change in the photo current can be detected. Peak heightand width of the photo detector signals depend on the diameter of theshadowing particle as well as on its drift velocity within the fluid F,i.e. on the flow velocity of the fluid F itself.

The size of the particle relative to the size of the apertures on thephoto detectors determines the magnitude of the shadowing effect. Alarge aperture size allows detecting large particles at the expense thatsmall particles produce a shadowing effect that might be too small todetect. A second light barrier 6 with a smaller aperture can increasethe shadowing effect of the smaller particles and thus make thesedetectable. Particles larger than the small-aperture diameter will bemisinterpreted as particles with the size of the small-diameter apertureand produce additional wrong counts. As the particle size spectranormally drop off the rapidly with increasing particle size, the numberof miscounts by this second light barrier 6 can be tolerated.

In order to enable quantitative particle size measurements, the flowvelocity of the fluid is measured by two ultrasonic transducers A, Bwith the first transducer A emitting in a direction parallel and thesecond transducer B emitting anti-parallel to the fluid flow. Bothtransducers A, B are in direct contact with the fluid. The fluidvelocity is determined using the time-of-flight measurement principle.This principle uses the difference in the sound propagation times fromthe emitting to the receiving transducer for sound emitted parallel andanti-parallel to the direction of the fluid flow to determine the flowvelocity. The measurement of the time of flight difference isschematically shown on the left hand side and on the bottom of FIG. 1.The function of the ultrasonic transducers A, B as transmitter G or asreceiver R is switched by a direction switch S. The measured timedifference between the upstream and downstream propagation times istaken as a measure of the fluid flow velocity. The speed of sound in thefluid F depends on the temperature, the fluid temperature needs to bedetermined for compensation purposes by a separate temperature sensor indirect contact with the fluid F.

FIGS. 2 a-2 c are an explanatory depiction of the functional principleof the device according to the present invention.

FIG. 2 a illustrates the shadowing effect of a single particle P passinga light barrier sensor with a velocity v_(fl) which is the flow velocityof the fluid. In this example the particle size is d_(p) and the size ofthe apertures on the photo detector is D_(W). LED's schematicallydepicted as arrows are used as light sources and photo diodes arepositioned opposite to the LED emitters. The diagram below the sketch inFIG. 2 a shows the current over the time, wherein Δt=d_(p)/v_(fl).

FIG. 2 b illustrates the generation of the output signal of the opticalsensor device by providing a schematic sketch of the electronic circuit.

FIG. 2 c shows a schematic output signal of the optical sensor, whereinthe output signal of a small particle is shown as a small rectangularfunction and wherein the output signal of a large size particle is shownas a large rectangular function.

It has to be mentioned that a small light beam/a small window diameteroptimizes the shadowing effect and the output signal.

Several embodiments, modifications and variations have been shown anddescribed to illustrate that the basic principles and inventive featuresof the preferred embodiment are contemplated to be used in further andwidely different applications according to the spirit and scope of theinvention.

REFERENCE NUMERALS

-   1 monitoring device-   2 metal casing-   3 optical emitter-   4 light path-   5 optical detector-   6 light barrier-   7 constant current-   8 optical window-   9 fluid carrying pipe-   A ultrasonic transducer-   B ultrasonic transducer-   F fluid-   G transmitter-   P particle-   R receiver-   S direction switch

1-11. (canceled)
 12. A device for monitoring particle contamination inflowing hydraulic fluids, comprising: means for particle counting andparticle sizing.
 13. A device for monitoring the particle contaminationaccording to claim 12, further comprising an optical sensor and a flowsensor.
 14. A device for monitoring the particle contamination accordingto claim 13, wherein the optical sensor is a photoelectric slot sensor.15. A device for monitoring the particle contamination according toclaim 14, wherein the optical sensor has a light beam diameter of 100μm.
 16. A device for monitoring the particle contamination according toclaim 13, wherein the flow sensor is an ultrasonic transducer.
 17. Adevice for monitoring the particle contamination according to claim 13,wherein two or more optical sensors having different light beamdiameters are provided.
 18. A method for monitoring particlecontamination in flowing hydraulic fluids, the method comprising:determining fluid flow velocity; counting particles in hydraulic fluidpassing a light barrier for a fixed period of time; and obtainingparticle size distribution by using a range of different trigger levels.19. A method according to claim 18, wherein repeated measurements ofparticle contamination are conducted to allow extrapolating the time atwhich a critical contamination level is likely to be reached.
 20. Amethod according to claim 18, wherein signal size and signal durationare attributed to particle size.
 21. A method according to claim 18,wherein a cumulative size spectrum and a differential size spectrum areused for calculating the particle size.
 22. A method according to claim18, wherein a range of detectable particle diameters is pre-set to 10 μmto 100 μm.